1. Field of the Invention
The invention relates to a process for preparing polyacrylonitrile and copolymers of acrylonitrile which are particularly suitable for processing into fibers.
The invention further relates to an active catalytic system for use in such polymerization and copolymerization reactions.
2. Description of the Prior Art
It is known that polyacrylonitrile and the copolymers of acrylonitrile containing the latter in a proportion of at least 85% by weight are widely used in the art, more particularly in the manufacture of fibers.
Such polymers are prepared in the art by polymerization of acrylonitrile or by copolymerization of acrylonitrile with one or more ethylenically unsaturated compounds copolymerizable therewith, using suitable catalysts and normally operating in an aqueous polymerization medium.
More particularly, useful catalysts include peroxide compounds generally, such as benzoyl peroxide and alkali metal or ammonium persulphates, or combinations of such compounds with substances of a reducing character, known as redox couples.
Other known catalytic systems comprise derivatives of hydroxylamine as oxidants in combination with reducing agents consisting of salts of metals in reduced form or inorganic compounds of tetravalent sulphur.
Known polymerization processes do not always lead to polymers of acrylonitrile of fully satisfactory properties.
Thus, for instance, when copolymerizing two or more monomers, difficulties are encountered in obtaining homogeneous polymerization products.
Moreover, various catalysts among those described above exhibit effectiveness in such a range of polymerization temperatures that the resulting polymers are of lower standards.
In other cases, improved polymers can be obtained only when polymerizing within such a range of temperatures that the reaction speeds are so low as to make the polymerization process impracticable or at least uneconomical.
On the other hand, it is known that the properties of fibers obtained from polymers and copolymers of acrylonitrile depend upon the properties conferred to the polymers or copolymers, such as molecular weight, distribution of molecular weights, structure and the like.
It was attempted in the art to attain such properties either by the choice of the catalytic system or by introducing special expedients in the polymerization process.
Thus, in accordance with certain known processes the polymerization is carried out in a plurality of steps in which the parameters, such as the pH and the temperature of the reaction medium, are maintained at different values, with the disadvantage of necessarily adjusting various parameters at the plurality of reaction steps.
Therefore, in preparing polymers and copolymers of acrylonitrile, problems arise relating to the progress of polymerization and also to the nature of the resulting polymers. In known processes, the requirements for inexpensiveness and simplicity of the process are hardly to be reconciled with those relating to imparting properties which make the polymers suitable for processing into high grade fibers.
The above drawbacks are avoided or at least minimized by the process of the invention which is essentially based on the use of a new catalytic system for the polymerization of acrylonitrile or copolymerization of the latter with further ethylenically unsaturated monomers.